The present invention relates generally to the area of thermal transfer printers. More specifically, the present invention relates to a thermal printer and a control process for thermal transfer printers that print on die-cut label media.
In general, the technology related to direct thermal and thermal transfer printers is well known in the prior art. Thermal transfer printers are designed for printing onto non-sensitized materials such as paper or plastic films. In the printing process, a transfer ribbon that includes a heat-transferable ink layer deposited on one side thereof is interposed between the media to be printed and a thermal print head that includes a row of very small, tightly spaced heater elements. To affect the transfer of the ink from the transfer ribbon to the media, an electrical pulse is applied to a selected subset of the heater elements within the printer head, thereby melting and transferring the ink adjacent the heater elements from the transfer ribbon onto the paper, resulting in a corresponding line of dots being transferred to the surface of the media. Since the print head is oriented horizontally with respect to the media, each time this process is repeated the printer prints one horizontal line onto the media. Generally in the art, thermal transfer printers also include more than one such thermal print head positioned adjacent and in spaced relation to one another, wherein each head corresponds to a separate color of ink. For example, many thermal transfer printers include either three heads for printing magenta, cyan and yellow inks or four heads for printing magenta, cyan, yellow and black inks.
In a similar fashion to the thermal transfer type printer, direct thermal printers print by utilizing small arrays of heaters to print directly onto sensitized materials. In a direct thermal printer, no transfer ribbon is used and the heater elements act directly with the sensitized media to produce chemical or physical change in a dye coating on the surface of the media. While the descriptions provided throughout this specification are directed primarily to thermal transfer printing, it should be appreciated that to the extent that similar features or constructions impact the printing process within other printing systems, those aspects of the present invention apply equally to equivalent technologies, such as those utilized in direct thermal printing.
After each respective line of dots is printed, the media is advanced slightly within the printer in order to position the print head over an adjacent location, the transfer ribbon is repositioned to expose a fresh coating of transfer ink and the heating process is repeated to print the next adjacent line of dots. Depending upon the number of print heads and the number of heaters on each print head, the printed arrays of dots can produce individual characters or images. Further, successive rows of dots are combined to form complete printed lines of text, bar codes, or graphics.
In order to print a coherent image, the printer must know at which points in time to activate the print head. Specifically, the printer needs to know the exact position of the media each time it activates the print head. In order to determine the position of the media relative to the print head, the printer utilizes an encoder that consists of a roller, which is engaged against the surface of the media. Every time the encoder roller rolls a specific amount, it sends an indexing signal to the print controller. Commonly the encoder is configured to notify the printer every time the media is advanced by 1/300th of an inch. Accordingly, each time the print controller receives a signal from the encoder, the print controller knows that it must print another line, thereby resulting in a printed line on the media every 1/300th of an inch.
It should be appreciated by one skilled in the art, that any particular ink transfer ribbon only has one ink on the transfer surface and accordingly is only capable of printing one shade of color no matter what heat intensity is utilized to transfer the ink from the ribbon to the media. Therefore, in order to create various shades or intensities of any given color, the printer utilizes a form of visual trickery known as half-toning. In the half-toning process, two approaches exist for varying the appearance of the dots in the printed output. In one approach, the printer controls the intensity of the heat utilized for the transfer of each of the individual ink dots to the media thereby controlling the actual size of each of the dots that are transferred. In this approach, as more heat is applied, a larger dot is produced and as less heat is applied, a smaller dot is generated. In a second approach, the printer divides the image into an array of virtual dots, each of which is formed from an array of individual pixels that each has a constant size. In this approach, the printer controls the size of the virtual dot by varying the number and pattern of pixels printed within the virtual dot. As can be appreciated, in this method, consistent dot size is critical to producing consistent print output. Accordingly, even though a thermal printer typically only has three colors, namely, magenta, cyan and yellow, any number of other colors can be created by overlying a half-tone print of each of the colors wherein the relative intensity level of each color is controlled by controlling the size of the dots by varying the heat to change the physical size of the dot or adjusting the number and pattern of pixels printed within a virtual dot. For illustration purposes, the following is a simple example of the half-toning process. Printing an image of solid magenta onto the media is easy because the printer includes a magenta transfer ribbon. All the printer has to do is fill the image on the media with magenta ink. When printing a light shade of magenta onto the media, the process becomes more complicated because the printer does not have a light magenta ribbon. To print a light magenta color, the printer must simulate it using the magenta print ribbon. Simulated lighter colors are created by controlling the size of the dots of magenta ink that are transferred, wherein the printer transfers relatively small dots (virtual or actual) of magenta ink and allows some of the original background color of the media to remain exposed. In this manner, the viewer's eye sees the mix of small magenta dots and the background color and perceives the overall mix as light magenta. To make an even lighter shade of magenta, the dots simply must be smaller in size thereby allowing more of the background color to show through the magenta ink. FIG. 1 more clearly illustrates the difference between a dark shade and a light shade transferred in this manner. Arrays of dots are shown wherein each dot is actually a virtual dot comprised of an array of individual pixels. The size of the dots illustrated in FIG. 1 has been exaggerated for clarity. Image 2 on the left has larger ink dots 4 where a large percentage of the pixels 5 within each of the virtual dots 4 are printed. Since the ink dots 4 cover more of the background color, the eye perceives this image 2 to be a dark shade. In contrast, image 6 on the right has smaller ink dots 8 where a smaller percentage of the pixels 9 within each of the virtual dots 8 are printed thus leaving more of the background color exposed and resulting in a perception of a lighter shade. When the printer prints the dots at the actual size of 1/300th of an inch, the eye does not see the individual dots but interprets the image as one solid color in a desired tonal shade (either a light shade or dark shade).
The difficulty found in this prior art printing method is that minute changes in the transport speed of the media through the print head result in undesirable fluctuations in the print head temperatures between print cycles. These small speed changes translate to visible artifacts in the image. Such artifacts often appear as an uneven transfer of ink from line to line in the image. The problem is further exacerbated when media, commonly known as gap media, is printed. Gap media is a continuous feed roll of sheet label media that is applied to a thin backing or liner sheet. The labels are die-cut from the label media and the border surrounding the cut labels is removed to create a series of individual labels attached to a continuous roll of liner material. A common gap media, for example, consists of 4″×6″ adhesive backed labels attached in series on a five-hundred (500) foot long roll of liner. The space between each of the labels is referred to as a gap. The particular feature of gap media that is problematic is that the leading edge of each label creates a lip that can catch on various mechanical parts on the interior of the printer. As the leading edge passes over and under the various mechanical parts of the printer, the speed of the media changes (typically slows), thereby further contributing to the creation of artifacts or uneven ink transfer.
It has been determined that while the actual interruption of the speed of the media may seem trivial when viewed in terms of actual transport speed, these minute interruptions result in visible banding within images. These bands are particularly pronounced when producing half-toned images. This problem can be better understood by reviewing a graph of the actual time spent printing and transporting the media relative to each count of the encoder. The graph shown in FIG. 2 details the relationship between the encoder count, which is represented along the X-axis 10 and the time in milliseconds spent printing a single line and advancing the media by 1/300th of an inch, which is represented along the Y-axis 12. The graph represents actual data collected when printing 4″×6″ labels having a continuous half-toned single color printed thereon. It is important to note that the graph does not represent media speed. Instead it represents the time period required to print a single line and advance the media one encoder step. In this particular graph, the printer was set to operate at a speed of 3 inches/second and a resolution of 300 lines per inch. Using these settings it follows that the printer operates by printing 900 lines/second, thereby requiring 1.111 milliseconds (mS) to print each line and advance the media to the next line. A review of the graph indeed confirms that the period between each encoder count falls generally between 1.1 mS and 1.2 mS.
What is more revealing about the graph however is that at fairly regular intervals, the time between encoder counts sharply jumps to nearly 1.4 mS. Further, in reviewing the particular locations of these extended line print times relative to the positioning of the media in the printer, there is a clear relationship between specific media positioning within the printer and the extended line print times. The portion of the graph between the bracket lines 14 and 16 represents the period of time wherein a sample label on the media roll was passing under the magenta print head. When comparing the peaks 15 that lie between the bracket lines 14, 16, it is clear that the peaks 15 correspond to physical positions on the printed label that are located 1.5″, 3.1″ and 4.4″ into the label. A sample of the label 18 that was printed while collecting the data as found in the graph clearly demonstrates that there is in fact banding 20 that occurs at each of the locations predicted by the extended print time peaks 15 shown in the graph.
In determining the reason for the appearance of the banding 20, the relative positioning of the gap media must be reviewed as compared to the mechanical components of the printer itself. This relationship between the media and the elements of the printer is illustrated in FIG. 4. Specifically, four print heads 22a, 22b, 22c and 22d are schematically shown. Gap media is illustrated having a liner 24 and labels 18 thereon. To understand the reason that the bands appear, an understanding behind the operation of the actual print head is necessary. A thermal print head does not instantaneously cool down the moment after it operates to affect a transfer of ink. Each time a line is printed, residual heat from the previous line remains within the print head. Circuitry within each of the print heads allows the system to reach a steady state by accounting for the history of each line that was previously printed. The limitation is that this circuitry only considers historical residual heat data and assumes a linear cooling rate and that the transport of the media is occurring at a steady and constant pace. The problem as was shown in the graph of FIG. 2 is that the assumption that the media is being transported at a constant state is an incorrect assumption. In fact it is clear that the transport of the media actually encounters regularly spaced periodic slowdowns. The reason for these slow downs can be seen in FIG. 4. As a label 18 passes under a print head such as the magenta print head 22d, the leading edge of the label 18 at some point must encounter the next print head 22c in the printer sequence. Due to the nature of the gap media, the media layer surrounding the die cut labels 18 has been removed from the liner 24 as was discussed above. This media format results in a small lip along the advancing edge of the label 18 that is susceptible to catching against the next print head 22c which it encounters, thereby creating a small mechanical drag on the media transport, which in turn causes a sudden and brief change in media transport speed. This is clearly the reason for such a media slow down because the banding as illustrated in the graph and on the sample label corresponds exactly to the relative spacing of the adjacent print heads within the printer used for testing.
The difficulty with the prior art is that in practice it has been demonstrated that the assumption of a constant media transport speed is incorrect. The impact of this incorrect assumption is clearly the appearance of banding each time the media transport speed is suddenly changed for any reason. Any time the media transport is briefly slowed, the time between the printing of one line and the next line is increased. In some cases this delay can be significant. Consider that the steady state duration is 1.1 mS and that the extended durations can be as much as 1.5 mS, which translates into an increase of time of as much as 36% between printing of adjacent lines. This extended duration allows the print head to cool down for a longer period of time before printing the next line. Remember that in the prior art the assumption is a constant transport speed. Therefore, this additional time that the print head is allowed to cool is not accounted for in the print process. As a result, if there is an abrupt slowdown in the transport speed, less residual heat will be present in the head and the array of pixels in the virtual dots that are printed immediately following the slow down will be smaller, thereby producing a row of virtual dots and therefore a line that looks lighter. Similarly, if the transport speed is abruptly faster, more residual heat will be present in the print head and the array of pixels in the virtual dots that are printed immediately following the speed increase will be larger, producing a line that looks darker.
In the prior art, there have been solutions introduced that attempt to solve the problem of inconsistent print quality during ramp up to a print operation. This is particularly a problem for certain types of “one-off” printers that frequently are required to print a single label or a single batch of a few labels and then wait in stand-by mode for the next set of instructions. In these cases, the media transport must accelerate in order to print the first label and decelerate during the printing of the last label. When operating in such a fashion, if the printer waited to begin the printing operation until the media transport reached the presumed constant state velocity, several unprinted labels would be wasted at the beginning and end of each batch job. This is the problem stated in U.S. Pat. No. 5,657,066 (Adams). In Adams, the controller accounts for instantaneous velocity during acceleration and deceleration and adjusts the pulse width of the strobe signal to maintain uniform print density during ramp-up and ramp-down periods at the beginning and end of each batch print job. However, the system in Adams still utilizes an assumption of smooth and consistent transport performance. Specifically, Adams assumes a constant acceleration, a constant state transport speed and a constant deceleration. Further, while Adams adjusts the print controller during acceleration and deceleration, it reverts to a constant transport speed assumption during normal operation. Accordingly, the Adams reference lacks the ability to overcome the periodic and subtle inconsistencies as identified above.
There is therefore a need for a thermal printer that includes a means for detecting minor and instantaneous changes in the transport speed of the media that is being printed and adjusting the printer strobe signal relative to such changes. Further, there is a need for a manner in which to control a thermal printer that detects and adjusts printer strobe signal durations instantaneously, based on precise feed back relative to actual media transport speeds between each encoder step thereby maintaining a reliably constant size for each and every printed pixel.